Recently, following development and practical realization of electric bicycles, electric carts, vehicles as various kinds of vehicle such as electric motorcycles, electric cars, etc. have become electrically motorized. Specifically, instead of cars which are powered by an internal combustion engine, hybrid cars which are powered by a gasoline engine and an electric motor; electric cars which are powered by an electric motor which is operated by a battery which is charged with household electric power supply or a charger which is provided at a gas station or an electric power supply station; or fuel cell cars which are powered by a fuel cell using hydrogen gas, etc. as fuel are developed in succession. Regarding the hybrid cars and the electric cars, a part of them are already practically used and are about to become widely used.
Conventional vehicles which are powered by an internal combustion engine such as a gasoline-powered car, a diesel powered car or a bike (hereinafter described as conventional car) generate an engine sound or an exhaust sound which is emitted from a power source itself and further generate a road noise, etc. while running. Therefore, pedestrians walking in the street or persons who get on a bicycle can perceive the approaching of a vehicle by an engine sound or an exhaust sound. However, in a case of hybrid cars, when the hybrid cars run at a low speed, they are not powered by an engine but mainly by an electric motor. Therefore, the engine sound or the exhaust sound is not generated. Further, in a case of electric cars or fuel cell cars, etc. they are driven by an electric motor in whole operation region. Therefore, the above-mentioned vehicles are electric vehicles that are excellent in quietness. However, pedestrians or persons who get on a bicycle in the vicinity of the electric vehicles which are excellent in quietness can not perceive the approaching of the electric vehicles such as a hybrid car powered by an electric motor which generates less sound and is excellent in quietness, an electric car, or a fuel cell car by sound. Consequently, accidental contact between the electric vehicles which are excellent in quietness and pedestrians, etc. may occur.
As above-mentioned, excellent quietness of hybrid cars, fuel cell cars, electric cars, etc. is advantage, however, the excellent quietness may sometimes be adverse effect. In order to solve the above-mentioned problem, in addition to a horn which is tooted by drivers of their own will so as to alert persons in the vicinity, various kinds of systems to notify the presence of the vehicle to the persons in the vicinity, without relation to intention of drivers of the vehicle, have been proposed.
For example, according to Patent Document 1, when a vehicle is started, a warning sound is generated, and the magnitude of the warning sound is changed to be high for the first 5 seconds, and then, lower. The warning sound on the low sound level is continued before the vehicle speed exceeds 10 km/h. In this case, the magnitude of the warning sound is gradually reduced as the time is elapsed, the volume of the warning sound is alternatively changed to be higher or lower, or the magnitude of the warning sound is changed according to the change of the vehicle speed.
According to Patent Document 2, an artificial engine sound signal having a frequency corresponding to the motor rotation number and an amplitude corresponding to an accelerator pedal position; and an artificial engine sound signal having a frequency corresponding to a vehicle speed which is detected by a vehicle speed sensor and an amplitude corresponding to the accelerator pedal position; are produced by a computer, and are output from a speaker through an amplifier. And it is selected by a switch such that whether the artificial engine sound depending on the motor rotation number is made, or the artificial engine sound depending on the vehicle speed is made. Further, it is mentioned such that an artificial engine sound including the frequency depending on the motor rotation number and the frequency depending on the car speed may be emitted